World's largest whale graveyard discovered in the Indian Ocean

At nearly 7,000 meters below the surface of the southeastern Indian Ocean, an international research team led by Chinese scientists has discovered the deepest and largest whale graveyard ever recorded — a 1,200-kilometer stretch of seafloor littered with whale remains that have accumulated over at least 5.3 million years.

The findings reveal that the region has served as a vast whale necropolis since the early Pliocene epoch, about 5.3 million years ago, hosting both active whale-fall ecosystems teeming with specialized deep-sea life and hundreds of fossilized cetacean skeletons.

The study, conducted by researchers from the Institute of Deep-sea Science and Engineering of the Chinese Academy of Sciences in Hainan province, the University of Pisa in Italy and Earth Sciences New Zealand, was published in the journal Nature on June 10.

The deep sea, a dark, high-pressure realm beyond 200 meters, hosts one of nature's most remarkable feats: whale falls. When a whale carcass sinks to the seafloor, it creates a self-sustaining ecosystem capable of sustaining communities of organisms for decades.

Previously, the deepest known whale fall was recorded at 4,204 meters in the southwest Atlantic Ocean. Moreover, almost nothing was known about whale-fall life below 6,000 meters, and the deep-sea fossil record of deep-diving whales remained blank.

Using the Fendouzhe manned submersible aboard the research vessel Tansuo 1, the team conducted 32 dives in early 2023 in the Diamantina Zone, a seafloor fracture belt stretching about 1,200 kilometers parallel to the Southeast Indian Ridge, and documented five active whale falls and 476 fossil cetacean sites along the corridor.

The bones were covered with dense white microbial mats and bone-eating worms, indicating they had remained on the seafloor for an extended period. The deepest active whale fall, found at 6,789 meters, consisted of three beaked-whale vertebrae, extending the known depth limit of whale-fall ecosystems by more than 2,500 meters.

"We did not set out to find whale falls or fossils, so this discovery was a wonderful surprise," Peng Xiaotong, a researcher at the institute and the study's first and corresponding author, said.

"But that is precisely the significance of hadal exploration — we are entering the most unknown ocean regions on this planet, and any extraordinary discovery could happen," Peng said.

The expedition was part of the Global Hadal Exploration Program, a 10-year United Nations Ocean Decade initiative launched by the institute. The program focuses on the hadal zone — ocean trenches and deeps below 6,000 meters that remain among the least explored environments on Earth.

Analysis of 43 fossil specimens identified five beaked-whale species and one baleen-whale species, including two living species still inhabiting the southeastern Indian Ocean today and two extinct genera previously known only from fossils trawled off South Africa. In addition, one specimen was identified as an entirely new species, named Pterocetus diamantinae.

Isotope dating of 33 fossil bones yielded ages from 5.26 million to 120,000 years ago, confirming continuous whale-fall events in the region since the early Pliocene of 5.3 million years ago.

The researchers said the concentration may be attributed to the fact that the trench serves as an important habitat for beaked whales, some of which are prone to death during deep dives when exceeding their physiological limits. Its V-shaped terrain facilitates the accumulation of carcasses at the trench bottom, while extremely low sedimentation rates allow whale bones to remain exposed on the seafloor for extended periods without burial.

Based on observed densities of 759.5 whale remains per square kilometer, the researchers estimated the area may contain more than 10 million cetacean carcasses. The total carbon sequestered in these whale remains amounts to about 6.7 million tons — equivalent to roughly 4,700 years of background marine snow accumulation in the region, the researchers estimated.

Stephen Godfrey, a researcher at the Calvert Marine Museum in Maryland, wrote in a companion commentary that the discovery "can be compared to many milestone discoveries in the history of deep-sea exploration" and is "a unique and major breakthrough".

Zhou Peng, an assistant research fellow at the institute, highlighted the important role of the Fendouzhe manned submersible in completing the study.

He said the manned vehicle offered an irreplaceable advantage for scientists to observe the environment firsthand and make real-time decisions about which specimens to collect.

"The manned platform provided by Fendouzhe opened up unprecedented space for this research and served as the key support behind the breakthrough results," Zhou said.